It is a routine medical procedure to insert a drainage catheter into a patient to remove excess fluid from a body cavity. For example, when excess fluid builds up in the pleural cavity, the fluid can prevent a lung from expanding normally, impairing breathing. Thoracentesis is a procedure to remove the fluid by placing a drainage catheter within the pleural space. During the procedure, several layers of body tissue must be penetrated to position the tip of the catheter, and the user must take care to avoid puncturing the lung itself.
Retractable centesis needle devices typically include a cannula with a sharp tip to cut through tissue walls and allow the device to be positioned within a body cavity. Some retractable centesis needles such as U.S. Pat. No. 4,447,235 include a retractable blunt needle positioned inside the cannula. In those devices, the blunt needle extends beyond the cutting edge of the cannula to prevent the user from inadvertently cutting or piercing through tissues during maneuvering. The action of the needle retracting may also provide visual, auditory, or haptic feedback when the blunt needle makes contact with tissue. The needle may be automatically refracted as the device is pressed against tissue by coupling the needle to a spring or other biasing member.
Retractable centesis needles may also include visual indicators. For example, the retractable needle device of U.S. Pat. No. 5,256,148 includes two regions colored in different colors. A section of the needle is viewable through a lens in the housing. As the needle is refracted, the colored portion of the needle visible in the lens changes. The change in color enables the user to estimate how far the needle has retracted into the sharp cannula while the device is obscured inside the patient's body.
Other devices seek to improve upon the design of U.S. Pat. No. 5,256,148 by improving the sensitivity of the indicating system. U.S. Pat. No. 6,447,483 describes a device that includes two opposing springs coupled to the retractable needle to bias it in opposite directions. The opposing-spring configuration is intended to decrease the initial resisting force at which the indicator is displaced as compared to a single spring of a similar size, alerting a physician more quickly when a tissue wall is contacted.
Although the initial force required to begin displacing the indicator is lower in the opposing spring system, the required force increases at a faster rate than the single spring alone. In other words, although the resisting force required to begin revealing the indicator is lower, a greater change in resisting force is required to reveal the entire indicator. A need therefore exists for a device that provides increased initial sensitivity until the indicator is revealed, and decreased sensitivity after the indicator is revealed. In addition, the springs in an opposing spring system work against one another, with the springs being in a compressed state even during a static state prior to use. The excess forces exerted inside the opposing spring device may weaken or deform the device components. Therefore, a need exists for a device that reduces spring forces during a static state prior to use.
The three US patents described above are herein incorporated by reference in their entirety.